In U.S. Pat. No. 3,080,693, issued Mar. 12, 1963 to William H. Glass et al, and assigned to the assignee of the present application, there is shown an automatic cleaning apparatus for a filter unit in a compressed air system that includes a purge reservoir, separated from a storage reservoir by a one-way flow valve device, from which purge reservoir substantially dry air flows back through a desiccant in the filter unit and a fluid pressure operated backflow vent valve device to atmosphere thereby reactivating the desiccant upon operation of this backflow vent valve device to its open or backflow venting position by fluid under pressure supplied thereto from the storage reservoir upon operation of a compressor governor to unload the compressor in response to the pressure in the storage reservoir reaching a first chosen value. In this filter unit reactivating apparatus the compressor governor maintains the communication between the storage reservoir and the backflow vent valve device open until the use of fluid under pressure from the storage reservoir reduces the pressure therein to a second chosen value, less than the above-mentioned first chosen value, whereupon operation of the governor, in response to this reduction of the pressure in the storage reservoir to this lower value, effects reloading of the compressor and the release of the operating fluid under pressure from the backflow vent valve device. Therefore, it is apparent that the backflow vent valve device remains in its open position for a period of time variable in accordance with the rate of use of fluid under pressure from the storage reservoir.
The substantially dry air under pressure, as it flows from the purge reservoir through the desiccant material in the filter unit picks up or entrains moisture from the desiccant, this moisture having been removed by the desiccant from the air previously supplied from the compressor to the storage reservoir as it flowed through the desiccant. It is well-known that as the pressure of a fluid is reduced, the temperature will be reduced a generally proportionate extent below the temperature of the ambient air surrounding the reservoir from which the fluid is flowing. Therefore, the temperature of the air from the purge reservoir having moisture entrained therein by reason of its backflow through the desiccant in the filter as it flows through the backflow vent valve device to atmosphere, will be reduced. This reduction of temperature of the flowing air will cause the entrained moisture to condense by flocculation into a flocculent mass that in winter weather would freeze. This icy mass would render the vent valve device inoperative thus preventing a subsequent reactivation of the desiccant in the filter unit. Moreover, with the vent valve device frozen in the open position, upon reloading of the compressor, some of the air compressed by the compressor, instead of flowing through the filter unit to the purge reservoir and thence to the storage reservoir, will flow past the open vent valve device to atmosphere. Of course such a loss of compressed air is undesirable.
Accordingly, it is the general purpose of the present invention to provide an automatic cleaning apparatus for a filter unit in a compressed air system that includes a purge reservoir, separated from a storage reservoir by a one-way flow valve device, from which purge reservoir fluid under pressure is supplied for effecting both reactivation of a desiccant in the filter unit and operation of a combined compressor unloading and backflow vent valve device through which air from the purge reservoir is released to atmosphere subsequent to passing through the filter unit. By this arrangement reclosure of the combined compressor unloading and backflow vent valve device is responsive to the comparative quick reduction of the pressure in the purge reservoir rather than to operation of the governor to effect restarting or reloading of the compressor, it being noted that this restarting or reloading operation of the governor is dependent upon the rate of use of the fluid under pressure in the storage reservoir which rate may vary considerably so that an appreciable amount of time could elapse before reclosure of the backflow valve device occurs thereby enabling the formation of ice therein in the manner explained above.